The present invention is related to liquid crystal cell optical devices and, in particular, to temperature compensation techniques for such devices.
Liquid crystal cells have applications in many fields. Common uses include displays for laptop computers and for wireless telephones, for example. In fiberoptic networks alone, liquid crystal cells may be used in many devices, such as variable optical attentuators (VOAs), polarization controllers, optical switches and switch matrices, phase shifter or retarders, tunable waveplates, and tunable filters.
One drawback to the wide-spread adoption of liquid crystal cell optical devices is that liquid crystal cell operations are affected by changes in temperature. Liquid crystal cells may have different liquid crystals and the light paths through the cells may be varied; but the operation of the cell is generally the same. Some output property, e.g., intensity, polarization state, phase, of the light transmitted through (or reflected by) the cell may be modified by a voltage applied across the cell. As temperature changes, the relationship between the output property and the applied voltage changes. This drawback is significant for optical networks because typically network components are widely scattered and often exposed to ambient temperatures.
To compensate for this temperature dependency, various techniques have been used. A common technique is an electrical circuit with a temperature-sensitive diode by which the applied voltage to the liquid crystal cell is controlled, as illustrated in FIG. 1A. The diode is assumed to have an output voltage response similar to that of the liquid crystal cell so that with a proper scaling of the diode output voltage, the voltage applied to the liquid crystal cell changes according to temperature to make the desired output property of the liquid crystal cell invariant with temperature changes. However, this is only a rough compensation for temperature changes and in many applications, such as typically found in optical networks, is not sufficient.
FIG. 1B illustrates a much more accurate technique. An optical feedback loop taps off a small amount of the output optical signal from the liquid crystal device for direct monitoring of the output. The feedback loop controls the voltage applied to the liquid crystal cell responsive to the monitored output signal. This provides for a very accurate control in response to temperature change, but is very costly because optical monitoring devices are expensive, and additional insertion loss is introduced.
Still another technique uses a table in which parametric data for temperature, optical output property, and applied voltage are stored for the liquid crystal cell device, as symbolically illustrated by FIG. 1C. In response to the measured temperature, the table yields the suitable voltage for application to the liquid crystal cell for the desired output. This technique has some serious drawbacks, however. First, it is time-consuming to build such a parametric table, which must be made for each cell. Secondly, if the cell has hysteresis, then the table is ineffective because not only is the relationship between the output property and applied voltage temperature dependent, but the relationship also depends upon the path by which these values are reached.
On the other hand, the present invention avoids the deficiencies of these techniques with an elegantly simple electrical feedback loop, which effectively renders the output property of the liquid crystal cell invariant under changes of temperature.
The present invention provides for a liquid crystal cell optical device which manipulates optical signals independently over a range of temperatures. The device has a liquid crystal cell, which receives the optical signals and manipulates the optical signals according to an output optical property, such as attenuation, responsive to an electrical signal, and a circuit arrangement which is connected to the liquid crystal cell. The circuit arrangement controls a current of the electrical signal to the liquid crystal cell with respect to temperature by predetermined control equations for the output optical property so that the device manipulates the optical signals independently of temperature. The circuit arrangement comprises a feedback circuit so that the current of the electrical signal follows the control equations, one of which is empirically determined with respect to temperature.